2 ways.
a. use caustic soda. buy from hardware shops. dilute a teaspoon of it into about 1 litre of tap water. rinse and clean up by just tap water. Remember, use goggles and gloves! becareful!
b. use ammonia solution, also from hardware shops, rinse direct on the dirt, clean up with water. also, have to put goggles and gloves on.
Former one, very powerful and easy. Must take precaution!!